At present, OLED display have been widely used in various types of display apparatuses, wherein it is understood that the luminance of an OLED corresponds to the driving current thereof and the driving current is controlled by a related driving transistor. However, because the driving transistors of pixels in a display apparatus may not have the same threshold voltage (Vth) due to the manufacturing process, the driving transistors may generate different driving currents and accordingly the OLEDs in different pixels may have different luminance. Hence, the display apparatus may have non-uniformity problem while displaying images. Thus, it is quite important to develop a pixel driving circuit capable of compensating the threshold voltage of driving transistor.